Psychologists unravel mystery of how we detect life

New research sheds light on how human beings visually detect the presence of a living being, even if it isn't immediately recognizable as animal or human.

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New research sheds light on how human beings visually detect the presence of a living being, even if it isn't immediately recognizable as animal or human. According to the findings, two specific movement cues must be present for this to happen -- movement needs to happen close to the ground and needs to mimic what we consider to be real gravitational acceleration.

"To survive, we have to be able to detect the presence of a living being in the visual environment -- regardless of whether it is a fellow human, a potentially dangerous predator, or even a prey animal," says Niko Troje, a professor in the Department of Psychology who researches visual perception and biological motion. "For that purpose, we need a way of detecting life that is independent of the particular shape of an animal or person."

Humans can correctly detect the movement direction of an upright computer-generated figure but struggle when the figure is turned upside down. This suggests that our visual systems are keyed in to some very specific life-detection cues.

Using simple computer-generated figures created from dots representing the major joints of the body, Dr. Troje's team set out to unravel what the human visual system is keying into for determining that a moving object is a living being.

They found that the visual system follows a clever strategy. If a single dot is moving in a clockwise circuit, the walker seems to face to the right. Likewise, if a dot moves in a counterclockwise circuit, the walker seems to face to the left. However, to really be accepted by the visual system as indicating the movement direction of a living being, the dots also need to be close to the ground and need to move in a way that mimics real gravitational acceleration.

Using only these two main cues, Dr. Troje's team created a point-light display which is perceived by viewers as a living being, despite the fact that it doesn't resemble any known human or animal shape and doesn't contain any other cues as to which direction it may be facing. The perception that the viewers have of looking at something alive is based wholly on the point-light display containing the two main cues that convey a clear sense of directionality to our visual system.

This research was conducted in collaboration with former Queen's graduate students Dorita Chang (University of Birmingham, UK) and Daniel Saunders (Harvard Medical School, Boston) as well as former postdoctoral fellow Masahiro Hirai (Institute for Developmental Research, Kasugai, Japan). These findings will be published in a forthcoming issue of Psychological Science.

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